1. Field of the Invention
The invention is related to an electric discharge wire-cutting machine in which a pulse-like voltage is applied between a wire electrode and an electrified workpiece, to discharge a pulse therebetween, by which the workpiece is processed. In particular, the invention is related to a wire electrode positioning control device of the electric discharge wire-cutting machine which feeds the wire electrode in a backward direction and positions the end of the broken wire electrode at a predetermined position when the wire electrode is broken.
2. Description of Related Art
In a general electric discharge wire-cutting machine, a wire electrode is installed along guides installed on the upper side and the lower side of a workpiece. The workpiece is the other electrode. It is located between the upper guide and the lower guide and confronting with the wire electrode. When the electricity is discharged between the wire electrode and the workpiece, which is the other electrode, the workpiece is processed to an arbitrary shape. During processing, the wire electrode may be broken in the workpiece unless the cutting conditions, i.e., the tension of the wire electrode and the wire feeding speed are appropriate.
In the case of breakage of the wire electrode, it is general practice in the prior art to re-install the wire electrode manually and then restart cutting. This is very troublesome and reduces the efficiency of the cutting operation. To improve this, there is an electric discharge wire-cutting machine provided with a mechanism for automating the re-installation of the wire electrode.
When the wire electrode is re-installed automatically, the wire electrode has to be fed to a wire electrode winding reel, which is installed downstream of the direction of supply of the wire electrode.
The installation is done by positioning the end of the wire electrode at a predetermined position and feeding the wire electrode in the direction of the wire electrode winding reel. If the wire electrode is not wound up on the wire winding reel after a predetermined time, the end of the wire electrode has to be again positioned at the predetermined position and the wire electrode is then re-installed. The causes of the error can be that the wire electrode is not well guided by the guide or the wire electrode becomes entangled while it is fed to the wire electrode winding reel. In order to automate the re-installation of the wire electrode, there is a demand for positioning the end of the wire electrode at a predetermined position. U.S. Pat. No. 4,412,118 discloses an electric discharge wire-cutting machine which can position the end of the broken wire electrode at a predetermined position. This electric discharge wire-cutting machine is explained by referring to FIG. 8.
The wire electrode 101 is fed from a wire bobbin 115 to a wire winding reel 102 during processing as shown in FIG. 8. The feeding direction of the wire electrode 101 is called the wire supplying direction (direction C). The reverse direction of the direction C is the winding direction of the wire electrode 101, and it is called the wire winding direction (direction D).
A workpiece 101 is located between the wire bobbin 115 and the wire winding reel 102. The electricity is discharged between the wire electrode 101 and the energized workpiece 106 and the workpiece 106 is processed.
An auxiliary wire feeding device 120 is installed on the side of the direction D from the workpiece 106. When the wire electrode 101 is broken, the auxiliary wire feeding device 120 positions the end of the wire electrode 101 at a predetermined position.
Both of a drive roller 111 and a pinch roller 110 of the auxiliary wire feeding device 120 can contact and separate from the wire electrode 101.
When the wire electrode 101 is fed in the direction C, the drive roller 111 and the pinch roller 110 are separated from the wire electrode 101.
If a breakage of the wire electrode 101 is detected by a breakage sensor 117, which is installed on the side of the direction C from the workpiece 106, the wire electrode 101 is sandwiched between the drive roller 111 and the pinch roller 110. A motor 112 rotates the drive roller 111 and the wire electrode 101 is fed in the direction D. When the end of the broken wire electrode 101 is detected by a wire end detecting sensor 109, a control device 119 stops the rotation of the motor 112. Therefore, the end of the broken wire electrode is positioned at a predetermined position.
In the above-mentioned electric discharge wire-cutting machine, the auxiliary wire feeding device 120 which consists of the motor 112, the drive roller 111, and the pinch roller 110 is necessary to feed the broken wire electrode 101 in the direction D. Therefore, there is a problem that the structure of the device is complex.
In this auxiliary wire feeding device 120, since the drive roller 111 and the pinch roller 110 have to be released from the wire electrode 101 to feed the wire electrode 101 in the direction C during processing of the workpiece, there is the problem that a releasing mechanism is also necessary.
In this auxiliary wire feeding device 120, when the broken wire electrode 101 is wound up, a sag in the wire electrode 101 is generated between the auxiliary wire feeding device 120 and the wire feeding reel 115 which supplies the wire electrode 101. Therefore, there is also a problem that the wire electrode 101, which is installed between the auxiliary wire feeding device 120 and the wire feeding reel 115, is derailed and it cannot be re-installed automatically.